Files entrusted to cloud-storage provider Dropbox were susceptible to unauthorised access via three attacks devised by security researchers, but the provider has since closed the vulnerabilities.

Files entrusted to cloud-storage provider Dropbox were susceptible to unauthorised access via three attacks devised by security researchers, but the provider has since closed the vulnerabilities.

Dropbox could also be used as a place to store documents clandestinely and retrieve them from any Dropbox account controlled by an attacker.

Researchers who presented their work at USENIX Security Symposium say they had developed the exploits last year but gave Dropbox time to fix the problems before making the exploits public.

First they managed to spoof hash values that are supposed to identify chunks of data stored in Dropbox's cloud. Dropbox checks these values to see if the chunks are already stored in the cloud and if so just links them to the account of the user who sent the hash.

By spoofing hashes, they were able to have Dropbox grant them access to arbitrary pieces of other customers' data, say the researchers from SBA Research in Austria. Since the unauthorised access was granted from the cloud, the customer whose files are being distributed didn't know it was happening, they say.

The second attack required stealing the victim's Dropbox host ID, which is a 128-bit key generated by Dropbox using such customer-specific factors as username, time and date. Once an attacker had a victim's host ID, he could replace his own with it. When he resynced his account, all the victim's files could be downloaded to him.

The third attack takes advantage of a feature that allows Dropbox customers to request file chunks via SSL at a certain URL. All that is necessary are the hash value of the chunk and any valid host ID - not necessarily the ID of the host with which the requested chunk is associated.

This last attack was detectable by Dropbox because of the mismatch between chunks requested and the accounts requesting them, the researchers say.

These three attacks could have been useful tools for stealing data from within organisations that used Dropbox, the researchers say. Rather than having to sneak entire files out of corporate networks, all attackers had to sneak out was the hash for the data they wanted. The hash could then be submitted to Dropbox from anywhere to download the actual data.

The attacks could have been used to hide data within Dropbox's cloud, the researchers say. Unlimited chunks of data could be uploaded to the cloud without being associated with the attacker's account by using a modified Dropbox client. To retrieve the data, attackers could send a hash of that data to Dropbox as if they intend to upload it. Since a chunk of data with a matching hash is already in the cloud, Dropbox would just link that chunk to the account sending the hash. Any account controlled by the attacker could access the data.

The implications for data thieves would be that they could upload files from a computer with no hard drive using a Linux live CD, leaving no traces for forensics experts to find on the computer.